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FACTS ABOUT NUCLEAR WEAPONS |
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Because of the tremendous amount of energy released in a nuclear detonation, temperatures of several crore degrees C develop in the immediate area (contrast this with the few thousand degrees of a conventional explosion). This compares with the highest temperature inside the core of the Sun. At these temperatures, every thing near ground-zero (a few hundred meters) vaporises. The remaining gases of the weapon, surrounding air and other material form a fireball. The fireball begins to grow rapidly and rise like a balloon. The combination of this rise, and subsequent expansion as it cools gives the appearance of the familiar mushroom cloud. The vapourised debris, contaminated by radioactivity, falls over a vast area after the explosion subsides--a deadly fallout with long term effects.
The energy of a nuclear explosion is released in the form of a blast wave, thermal radiation (heat) and nuclear radiation. The distribution of energy in these three forms depends on the yield of the weapon. For nuclear weapons in the kiloton range (as in Pokhran II), the energy is divided in various forms, roughly as 50% blast, 35% thermal and 15% nuclear radiation. Each one of these forms causes devastation on a scale that is unimaginable. Below we discuss these effects separately for a 15 kiloton bomb.
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Because of the very high temperatures and pressures at ground zero, the gaseous residues of the explosion move outward. The effect of these high pressures is to create a blast wave travelling several times faster than sound. The pressure created is in excess of 10 Psi (pounds per square inch) with wind speeds in excess of 800 kmph up to about 1.2 km radius. Most buildings are demolished and there will be almost no survivors. This is the real killing field. Beyond this distance, and up to about 2.5 km the pressure gradually drops to 3 Psi and the wind speed comes down to about 150 km as in a severe cyclonic storm. There will be injuries on a large scale and some fatalities. Beyond this zone of fatalities, the pressure drops to less than 1 Psi, enough to shatter windows and cause serious injuries. It is the high speed combined with high pressures which causes the most mechanical damage in a nuclear explosion. Human beings are quite resistant to pressure, but not to being thrown over against hard objects nor to buildings falling on them. This damage is clearly more serious in built-up areas.
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The surface of the fireball also emits large amounts of infrared, visible and ultraviolet rays in the first few seconds. This thermal radiation travels outward at the speed of light. As a result this is by far the most widespread of all the effects in a nuclear explosion and occurs even at distances where blast and nuclear radiation effects are minimal.
The intensity of the thermal radiation can exceed 1000 Watts per square cm. This is similar to getting burnt by an acetylene torch used for welding metals. For a 15 kiloton bomb, almost everybody up to a distance of 2 km will suffer third degree burns (which damage the skin and tissues below it). There will be almost no survivors since no immediate medical attention will be available. Because the radiation is so intense, direct viewing of the fireball causes injury to the eye. Beyond this region people suffer from second degree burns, up to about 2.5 km, which is normally serious but may not be fatal. The effects persist even up to 3.5 km, with people suffering first degree burns (such as redness of the skin and pain).
The intense heat will also cause the burning of petrol, diesel and other fossil fuels along with wood, paper, fabrics etc. This could start a tremendous firestorm as happened in Hiroshima.
While blast and thermal effects occur to a far lesser degree in other types of explosions, the release of intense ionising radiation is a phenomenon unique to nuclear explosions. The ionising radiation as opposed to thermal radiation consists of fast moving neutrons, gamma rays, electrons and alpha particles. All these have the effect of creating chemically active free radicals (molecules breaking into ionising fragments) in living beings. This affects the normal behaviour of living cells. Furthermore, the initial radiation makes the surrounding atoms (of the air, soil etc.) radioactive and they in turn could emit nuclear radiation over an extended period of time from a few minutes to a few years.
Radiation injury has a long-term effect on survivors. Reactive chemicals released by ionization cause damage to DNA and disrupt cells by producing immediate effects on metabolic and replication processes. While cells can repair a great deal of the genetic damage, that takes time, and repeated injuries make it that much more difficult. Immediate treatment requires continual replacement of blood so that the damaged blood cells are replaced, and treatment of bone marrow and lymphatic tissues which are amongst the most sensitive to radiation. One must remember in this context that there are very few hospitals equipped to carry out such remedial procedures.
Radiation injury is measured in a unit called rem. Some authorities consider 5 rem/year tolerable for workers who are occupationally exposed to radiation --a typical value for exposure to medical X-rays is 0.08 rem. 1.5 rem/year is considered tolerable for pregnant women. It should be remembered that natural radiation is always present in the atmosphere over most places on the earth, but at lower levels. However, there is no threshold, universally agreed upon, at which a dose of radiation can be declared safe.
In the case of a nuclear explosion the radiation levels, up to several hundred metres away from ground zero, are a few thousand times that of what one normally receives from the environment or for medical purposes.
That is not all. Things which get irradiated by this ``prompt'' radiation themselves become radioactive and people in the area of a nuclear explosion, who are exposed to these radioactive materials, stand more risk of contracting cancer. A 1000 rem exposure for the whole body over a lifetime (which is entirely possible) brings about an 80% chance of contracting cancer.
Ionising radiation from the fireball produces intense currents and electromagnetic fields, usually referred to as the electromagnetic pulse (EMP). This pulse is felt over very large distances. Electrical grids will be subjected to voltage surges far exceeding those caused by lightning. Modern VLSI chips, present in most communication equipment. TVs, radios, computers and other electronic equipment are extremely sensitive to these surges and immediately get burnt out. So all possible communication links to the outside world are cut off. Restoring these facilities will be an arduous (and expensive) task.
There are also long term effects on the atmosphere and climate. These are not as direct as the fallout. The high temperature of the fireball can cause large amounts of nitrogen oxides to form during the cooling process, causing a depletion of the ozone layer in the stratosphere.
The famous proposal of a potential nuclear winter cannot be easily ruled out. This effect is caused when a large amount of soot is emitted during the burning of various material like petroleum. The soot effectively blocks sunlight, affecting the climatic conditions over a macroscopic domain. When the volcano Tambora (on the island of Sumbawa in Indonesia) erupted in 1815, it threw out so much soot that the next year was a ``year without a summer'' as far away as Europe and America--the coldest year in a few centuries. Although the proposal of nuclear winter was initially received with some skepticism, later studies have confirmed almost all the details. An immediate effect is the decrease in food production since most of the food in the world is produced in subtropical regions, leading to famine, starvation deaths, etc. Smaller weapons may not produce a nuclear winter, but a mild `nuclear autumn' cannot be ruled out.
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